Monitor refresh rate changes after flicker

I have a librem 13v4 and a ThinkVision p27h-10 monitor connected via HDMI. Pureboot-beta-13.1 on PureOS.

If I set the external monitor resolution to 2560 etc, immediately after I can see from the monitor info that it uses 60Hz refresh rate. However, suddenly the screen flickers and when it comes back (still the same resolution) it has changed to 58 something or even sometimes 59 something. After this the monitor flickers off and on once in a while, sometimes remaining black for several seconds. Sometimes it is stable for 30-40 minutes before flickering again. If I change to a lower resolution (1920 etc) this does not happen - everything is fine.

Anyone have any idea? I have changed cables and I am sure the cables are rated for this resolution/bandwidth.

I saw that I could not set the refresh rate of the external monitor but I could change the internal librem refresh rate from 60 to 59.96Hz. I tried changing the internal refresh rate to that lower number and then switching the external monitor to 2560 again, and now it hasn’t flickered once in 20 minutes, which is already an improvement. Weird?

Oh, and using stock PureOS, so this is Wayland, not x.

…and now the flickering is back so that didn’t work. Intermittent blanking of screen is maybe a better description…